Electrical coupling providing transition between a microstrip line and a perpendicularly oriented waveguide is often needed for high radio frequency system integration. A typical such coupling arrangement is shown in FIGS. 1 and 2. A microstrip line 10 formed on an upper surface of a dielectric substrate 20 ends in a probe 12. A metallic layer 26 on the opposite, lower surface of substrate 20 provides a ground layer for microstrip line 10. A waveguide 30 has an end 32 attached to the upper surface of substrate 20 surrounding the probe; and a wall opening 34 in waveguide 30 adjacent substrate 20 provides access to the interior of the waveguide for microstrip line 10.
A quarter wavelength shorting cap 40 is attached to metallic layer 26 below the lower surface of substrate 20 directly under waveguide 30. Shorting cap 40 is coupled to waveguide 30 by a plurality of parallel conductors, including conductors 52, 54 and 56 as representative examples, forming a via fence through substrate 20 and the removal of the portion of metallic layer 26 within the via fence. Probe 12 is made as narrow as possible to minimize blockage of energy flow between the waveguide and shorting cap 40. Shorting cap 40 ensures that the TE10 mode electric field maximum occurs coincident with probe 12 for efficient energy transfer. But shorting cap 40 adds cost and occupies space that may be needed in some packages for other components.